NestJS

NestJS

Overview

Enterprise Node.js architecture standard using NestJS, TypeScript, and RxJS. Enforces strict modularity, dependency injection, repository pattern, DTO validation via class-validator, and clean layered architecture.

When to Use

Activate when building enterprise Node.js microservices, complex REST/GraphQL APIs, or scalable backends requiring strict architectural structure.

Rules & Patterns

NestJS — Best Practices

Module Architecture

• One module per domain — UsersModule, AuthModule, OrdersModule
• Feature modules — encapsulate related controllers, services, repositories
• Shared module — for cross-cutting concerns (logging, config, utils)
• Core module — singleton services (database, auth guards)

src/
├── modules/
│   ├── users/
│   │   ├── users.module.ts
│   │   ├── users.controller.ts
│   │   ├── users.service.ts
│   │   ├── users.repository.ts
│   │   ├── dto/
│   │   │   ├── create-user.dto.ts
│   │   │   └── update-user.dto.ts
│   │   ├── entities/
│   │   │   └── user.entity.ts
│   │   └── users.spec.ts
│   └── auth/
├── shared/
│   ├── guards/
│   ├── interceptors/
│   ├── pipes/
│   └── filters/
├── config/
└── app.module.ts

Dependency Injection

@Injectable()
export class UsersService {
  constructor(
    @InjectRepository(User) private readonly usersRepo: Repository<User>,
    private readonly configService: ConfigService,
  ) {}
}

• Prefer constructor injection
• Use custom providers for complex setup
• Scope: default is Singleton, use REQUEST scope only when needed

DTOs and Validation

import { IsEmail, IsString, MinLength } from 'class-validator';

export class CreateUserDto {
  @IsEmail()
  email: string;

  @IsString()
  @MinLength(2)
  name: string;
}

• Always use DTOs for request validation
• Use ValidationPipe globally
• Separate Create/Update/Response DTOs

Guards, Interceptors, Pipes

Execution order: Guards → Interceptors → Pipes → Handler → Interceptors → Filters

Error Handling

@Catch()
export class AllExceptionsFilter implements ExceptionFilter {
  catch(exception: unknown, host: ArgumentsHost) {
    // Transform to standard error format
  }
}

Testing

• Unit tests — mock dependencies with Test.createTestingModule()
• E2E tests — use supertest with a test module
• Mock everything — services should be testable in isolation

Anti-Patterns

• [FAIL] Business logic in controllers — use services
• [FAIL] Direct database access in controllers — use repositories
• [FAIL] Circular dependencies — refactor module structure
• [FAIL] God modules — split large modules by domain
• [FAIL] Not using DTOs — always validate input

Code Examples

See EXAMPLES.md for detailed code examples.

Validation Checklist

What to verify during the review phase before completing the task.

Common Mistakes

Anti-patterns and things to explicitly avoid. See TROUBLESHOOTING.md.

Integration Notes

How this skill interacts with other skills.

nestjs Examples — Anti-patterns vs ContextOS Standard

Example 1: Input Validation and DTOs

Anti-pattern: Untyped Body or Manual Validation in Controller

// BAD: No runtime validation, controller stuffed with business rules
@Post('users')
async create(@Body() body: any) {
  if (!body.email || !body.email.includes('@')) {
    throw new BadRequestException('Invalid email');
  }
  return this.usersService.create(body);
}

Best practice: ContextOS Standard (Class-Validator DTO + ValidationPipe)

// GOOD: Declarative runtime validation with clean separation
export class CreateUserDto {
  @IsEmail({}, { message: 'A valid email is required' })
  email: string;

  @IsString()
  @MinLength(8, { message: 'Password must be at least 8 characters long' })
  password: string;
}

@Controller('users')
export class UsersController {
  constructor(private readonly usersService: UsersService) {}

  @Post()
  async create(@Body() dto: CreateUserDto) {
    return this.usersService.create(dto);
  }
}

nestjs Troubleshooting & Common Mistakes

1. Circular Dependency Between Modules

• Symptom: "Nest cannot create the module instance. Often, this is caused by circular dependencies".
• Root Cause: Module A imports Module B, and Module B imports Module A.
• Fix: Use forwardRef(() => ModuleB) in imports and @Inject(forwardRef(() => ServiceB)) in constructors, or refactor shared logic into a separate CommonModule.

2. Memory Leaks from REQUEST Scope

• Symptom: High memory usage and slow performance under load.
• Root Cause: Providers declared with Scope.REQUEST recreate instances on every HTTP request.
• Fix: Keep services as default Singletons whenever possible. Pass request-scoped parameters directly through method arguments.

3. Uncaught Domain Exceptions

• Symptom: Custom domain exceptions bypass formatting and return generic 500 errors.
• Root Cause: Missing custom Global Exception Filter.
• Fix: Implement an AllExceptionsFilter implementing ExceptionFilter and bind it globally in main.ts.